Shaping of composite materials, such as pre-preg (layers of fibre material previously impregnated with resin), into details is today commonly performed by thermoforming. In thermoforming, a blank of composite material is placed on a forming tool so that a part of the composite material protrudes over the edge of the tool. The tool and the composite material are covered with a vacuum foil, such as a rubber sheet which is sealed air-tight around the tool. The arrangement is heated and the air under the vacuum foil is sucked out. Due to the increased temperature the composite material softens and is shaped over the forming tool by the force exerted by the vacuum foil exerted on the surface of the protruding part of the blank. The shaped article is thereafter placed in its green state in a curing oven for curing. Examples of related technology are presented in the documents EP1136236 A1, EP1609584 A1 and U.S. Pat. No. 4,548,859 A1.
When using the aforementioned method for shaping of complex composite articles, such as elongated C-shaped beams with longitudinal flanges, a common problem is that the material does not closely follow the contour of the forming tool as the material forms over tool. That is, the radius of the material that is bent over the edge of the tool becomes larger than the radius of the edge of the forming tool, the so called Mickey-mouse effect. This effect is comes due to the fact that the pressure from the vacuum foil is too low at the beginning of the forming process. The problem is even larger for heavy blanks which tend to self form over the tool due to their own weight at the beginning of the forming process.
A further problem is that creases may arise in the articles, due to friction between fibre layers in the blank as the layers slide on each other during forming of the blank. In the case of shaping composite materials which comprise thermo-particles for increased impact strength, the tendency of creasing is even greater since the thermo-particles further increases the friction between the layers.
Previous attempts for solving the above mentioned problems have included manual lay-up and forming over the tool. The use of manual labour for laying-up and forming the material over the tool is time consuming and involves high costs. Another known method for solving the aforementioned problems is to support the composite blank with a release sheet during forming. The disadvantage of this solution is that further forming of the article cannot be done, and that the release-sheet itself can be an incitement for the creation of creases in the composite.
It is an object of the present invention to provide an improved shaping of composite materials into complex forms. This is achieved by a method being defined in the introduction. It is also an object of the present invention to provide a device for shaping a composite article. This is achieved by a device being defined in the introduction.